When drilling oil or gas wells, it is frequently necessary to send electrical cables down the hollow drill string for the purposes of supplying instruments for monitoring the position and orientation of the drilling assembly. Under previous practices the cables were passed down the hollow interior of the pipes making up the drill string. If it was desired to continue drilling with the cable in place, it was necessary to withdraw the cable everytime a new section of drill pipe was to be added to the drill string. Such withdrawal of the cable was time consuming and costly.
One approach to this problem is to insert a wireline cable through the side of a section of drill pipe. The cable is connected to a cablehead assembly which is attached to the selected instrument.
U.S. Pat. No. 3,804,168 to Marshall and Myska disclosed a wireline clamp which is installed outside the well and the upper end of the drill string.
U.S. Pat. No. 4,200,297 to Tricon disclosed an arrangement where a wireline cable entered the side of a drill string through a side entry sub. The cable entering the sub negotiated a curvature which was too sharp to permit insertion of a cablehead assembly through the side entrance.
U.S. Pat. No. 4,062,551 to Base describes a side entry sub where a clamp is attached to the body of the sub. A wire packoff is integrated into the side entrance to seal against loss of drilling fluid pressure through the sub. The orifice receiving the packoff is made as narrow as possible so that the problem of guarding against pressure loss is minimized. The orifice is not wide enough to receive the cablehead assembly. Consequently the cablehead assembly cannot be passed through the side entry sub but instead must be attached to the wireline from inside the drill string.
There are several disadvantages to the above devices. In no case can a cablehead assembly be passed through a side entry sub. Only plain wireline can pass through the side entrance into the drill string. Consequently, the cablehead must be attached to the wireline on the rig floor after threading the wireline through the side entry sub. This operation must at times be performed in adverse climatic conditions and uncontrolled environments of cleanliness. The makeup and soldering of electrical connections can be inferior. Moreover, the time element required for fabricating laboratory or service shop quality connections on the rig floor reduces control of the well and increases cost of operations.
Another problem with the above devices is that the outside diameter of the side entry sub is typically not concentric, or not of the same diameter as the associated tool joints, or it has localized protrusions above standard API tool joint dimensions.
A third problem occurs because the transition radii of the wireline in passing from the outside of the drill string to the inside are generally small, the passing angles being on the order of about 15.degree.. This increases wireline wear.
A major problem with the Base patent is that the wire must be packed off before the wireline is clamped. This sequence enables line movement and damage to the packings before on-line operations can begin. Thus possible leaks and washouts are threatened. Similarly, when coming out of the hole and terminating wireline operations, the cablehead in the Base patent must be cut off the wireline.
There is need for arrangements and procedures for introducing wireline cable into the side of a drill string which enable a cablehead assembly to be passed through the point of entrance.